1. Field of the Invention
The present invention generally relates to bearing assemblies. In particular the present invention relates to x-ray tube bearing assemblies having bearing surfaces that feature a thermally and mechanically stable lubricating coating that reduces flaking and spalling, thereby enhancing x-ray tube performance and longevity.
2. The Related Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination and therapeutic radiology, semiconductor manufacture and fabrication, and materials analysis.
Regardless of the applications in which they are employed, x-ray devices operate in similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an evacuated x-ray tube.
The evacuated x-ray tube typically includes a first housing portion that forms an evacuated enclosure. Typically, the evacuated enclosure is constructed of a heat-conductive material, such as copper. However, various materials, and combinations of materials, can be used.
Disposed within the evacuated enclosure is a cathode, or electron source, and an anode oriented to receive electrons emitted by the cathode. The anode can be stationary within the tube, or can be in the form of a rotating annular disk mounted to a rotor shaft that is rotatably supported by a bearing surface contained in a bearing assembly. The bearing assembly generally comprises a housing, a shaft, and ball bearing sets oriented to provide bearing surfaces for rotatably supporting the shaft. Each ball bearing set typically comprises several spherical ball bearings that are disposed between circular tracks, called races. The ball bearings are constrained to roll in the races between the bearing housing and bearing shaft, thereby enabling the bearing shaft to rotate within the housing. The rotor shaft, in turn, is operably connected to the bearing shaft so as to enable the rotor shaft and anode to rotate. The rotating anode, rotor shaft, and bearing assembly are therefore interconnected and comprise a few of the primary components of the rotor assembly.
In operation, an electric current is supplied to a filament portion of the cathode, which causes a stream of electrons to be emitted by thermionic emission. A high voltage potential placed between the cathode and anode causes the electrons to form a stream and accelerate toward a target surface located on the anode. Upon approaching and striking the target surface, some of the resulting kinetic energy is released in the form of electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials with high atomic numbers (“Z numbers”) are typically employed. The x-rays are then collimated so that they exit the x-ray tube through a window in the tube, and enter the x-ray subject, such as a medical patient.
As discussed above, some of the kinetic energy resulting from the collision with the target surface results in the production of x-rays. However, much of the kinetic energy is released in the form of heat. Still other electrons simply rebound from the target surface and strike other “non-target” surfaces within the x-ray tube. These are often referred to as “backscatter” electrons. These backscatter electrons retain a significant amount of kinetic energy after rebounding, and when they also impact other non-target surfaces they impart large amounts of heat.
A substantial amount of heat generated from these target and non-target electron interactions is transmitted via the anode and rotor shaft to the bearing assembly and the ball bearing sets. If not manufactured to adequately withstand these temperatures, the one or more ball bearings that comprise the ball bearing set may prematurely fail, rendering the x-ray tube inoperable.
In addition to high temperatures, the ball bearings are also subjected to significant mechanical stress imposed by virtue of the high rate of rotation experienced by the ball bearing sets during tube operation. In some high powered tubes, for instance, the rotating anode, which is supported by the bearing assembly, may rotate at rates exceeding 3,600 revolutions per minute (“rpm”). The ball bearings must be strong enough to withstand the stress imposed by this rotation. If not, premature bearing failure is the likely result.
One way to alleviate the effects of the thermal and mechanical stress discussed above is through lubrication of the ball bearings. In addition to improving ball bearing performance, lubrication also reduces noise and friction within the ball bearing sets, which helps to ensure smooth anode rotation and proper tube operation. Because the bearing assembly is disposed within the vacuum environment of the tube's evacuated enclosure, however, traditional bearing lubricants, such as grease or graphite, may not be used. Instead, ball bearings disposed in such vacuum environments typically comprise a metallic core, such as steel, onto which a hardened lubricating coating is directly applied. The lubricating coating may comprise a metallic material, such as silver.
The lubricating coating has typically been applied directly to the ball bearing core using one of several methods. One method is known as potential-driven ion deposition. In this process, the ball bearing cores are placed in a charged gas chamber containing argon or similar gas. Metallic ions that will comprise the lubricating coating are driven by an electric current to impact the metallic ball cores, thereby plating the cores with a metallic coating. This process is frequently used to apply a lubricating coating of lead on the ball bearing cores.
The ion deposition process as described above suffers from several drawbacks, however. Lead, which is typically used in this process to form the lubricating coating, is thermally unstable and tends to dissipate at high temperatures. Thus, the lubricating coating may undesirably evaporate from the surface of the ball bearings during periods of tube operation where high temperatures are present within the bearing assembly. The lead gradually dissipates until the lubricating coating completely disappears and failure of the ball bearing set occurs. Though the ion deposition process has been used to attempt the application of materials other than lead directly to the ball bearing core, these attempts have proven unsatisfactory. For example, silver, which is a popular lubricating coating, has not been successfully applied directly to ball bearing cores using ion deposition because of the low adhesion that exists between silver and steel.
Another method that has been attempted in applying the lubricating coating to the metallic ball bearing core has involved a mechanical bonding process. In this process, a lubricating coating, typically comprising silver, is electroplated directly to the ball bearing core, which as described before, typically comprises steel or similar metallic substance. This forms a mechanical bond between the silver coating and the metallic core. However, several problems result from ball bearings made in this manner. First, the mechanical bond existing between the metallic core and the lubricating coating is a relatively low-strength bond. The relative weakness of the mechanical bond may cause the lubricating coating to flake away from the ball bearing when it is subjected to relatively small amounts of thermal or mechanical stress. As already discussed, an x-ray tube bearing assembly is a high vacuum environment, full of thermal and mechanical stress. Thus, this environment can readily cause a mechanically bonded lubricating coating to flake away from the ball bearing surface. This debris can then contaminate the ball bearing set, significantly increasing friction and wear within the ball bearing set, and leading to its premature failure.
The above situation is made worse when ball bearings having mechanically bonded lubricating coatings are employed in high power x-ray tubes. These high power x-ray tubes are capable of higher operating temperatures and longer operating times than standard x-ray tubes. This, in turn, results in increased mechanical and thermal stress of the bearing assembly and the ball bearing sets. Unfortunately, this additional stress serves only to increase the incidence of flaking of the ball bearing lubricating coatings.
In addition to the above problems, flaking of the lubricating coating from the ball bearing surface in x-ray tubes may also result in electrical arcing within the high electrical potential vacuum enclosure. This electrical arcing can cause severe electrical damage to and/or failure of x-ray tube components, and should be avoided.
Another drawback encountered with mechanically bonded lubricating coatings relates to the preparation work required to apply the lubricating coating during manufacture of the ball bearings. Before mechanically bonding the lubricating coating to the outer surface of the ball bearings, grit blasting of the metallic core is often necessary in order to prepare the outer surface for adhesion of the lubricating coating. In grit blasting, the surface of the ball bearings to be coated is blasted with high velocity bits of material, such as silicon oxide or other suitable material, in order to increase the surface area to enhance the adhesion of the coating to the surface. While effective at preparing the ball bearing core surface, grit blasting may also temporarily embed grits into the surface of the ball bearing. Later, during operation of the x-ray tube, these grits may work free from the ball bearing and contaminate the ball bearing set. As was the case with the flaking of the lubricating coating, this grit contamination significantly increases the friction and wear of the ball bearing set, and often results in premature failure of the bearing assembly.
Additionally, other problems exist when grit blasting is used to prepare the ball bearing surface for a mechanically bonded lubricating coating. Because of their relatively small size, ball bearings are difficult to uniformly grit blast. This results in ball bearings having portions of the outer surface where little or no grit blasting has occurred, thus preventing an adequate footprint to be formed in these areas. This results in poor adhesion between the outer surface of the ball bearing core and the lubricating coating that is applied thereon. Thus, the likelihood of flaking of the lubricating coating from such areas is significantly increased. The undesirable effects of such flaking have already been described.
Finally, because the surface of the ball bearing core is roughened by the grit blasting before mechanically applying the lubricating coating, the resulting coating possesses a less than optimally smooth surface. This undesirably results in greater noise and vibration with the bearing assembly during tube operation, which detracts from the overall performance of the x-ray tube.
What is needed, therefore, is a ball bearing that is manufactured so as to avoid the problems described above. In particular, a need exists for a ball bearing having a lubricating coating that is thermally and mechanically stable in high vacuum, high heat, and high rotational environments, such as those encountered in x-ray tubes. Such a lubricating coating should enable smooth operation of the ball bearing set within the bearing assembly. Further, the ball bearing should not suffer from flaking or other problems associated with previous techniques that apply the lubricating coating directly to the ball bearing core.